Method of nano-patterning a foil surce

ABSTRACT

A method for patterning a metal substrate includes a series of surface treatments to control tunnel initiation at a micron or sub-micron level. In particular, the series of surface treatments include forming a hydration layer which acts as a mask while etching the surface of the metal substrate. The hydration layer mask enables control of the tunnel initiation on a micron or sub-micron level because the etching does not undercut the interface between the metal substrate and the hydration layer. As a result, the tunnels can be initiated in an orthogonal direction and closer together, thereby increasing the tunnel density.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to patterning of a foil surface. Moreparticularly, the present invention relates to etching an aluminum foilfor use in electrolytic capacitors.

2. Related Art

Electrolytic capacitors are compact, high voltage energy storage meansused in many applications, including implantable medical devices. Forexample, Implantable Cardioverter Defibrillators (ICDs) conventionallyinclude electrolytic capacitors because such capacitors have a highenergy density and can withstand a relatively high voltage, ICDstypically use two electrolytic capacitors in series to achieve thedesired high voltage for shock delivery. For example, an ICD can utilizetwo 250 to 500 volt electrolytic capacitors in series to achieve avoltage of 500 to 1,000 volts.

Conventionally, an electrolytic capacitor includes an etched aluminumfoil anode, an aluminum foil or film cathode, and an interposed kraftpaper or fabric gauze separator impregnated with a solvent-based liquidelectrolyte. The electrolyte impregnated in the separator functions asthe cathode in continuity with the cathode foil, while an oxide layer onthe anode foil functions as the dielectric. The entire laminate isrolled up into the form of a substantially cylindrical body, or woundroll, that is held together with adhesive tape and is encased, with theaid of suitable insulation, in an aluminum tube or canister. Connectionsto the anode and the cathode are made via tabs. Alternative flatconstructions for aluminum electrolytic capacitors are also known,composing a planar, layered, stack structure of electrode materials withseparators interposed therebetween.

These capacitors must typically store approximately 10-100 joules.Because the capacitance of an electrolytic capacitor increases with thesurface area of its electrodes, increasing the surface area of thealuminum anode foil results in increased capacitance per unit volume ofthe electrolytic capacitor. Thus, their size can be relatively large,and it can be difficult to package them in a small implantable device.Currently available ICDs are relatively large devices (over 20 to 40cubic centimeters (cc)), generally about 12-16 millimeters (mm) thick. Apatient who has a device implanted can often be bothered by the presenceof the large object in his or her pectoral region. For the comfort ofthe patient, it is desirable to make smaller ICDs. The size andconfiguration of the capacitors contribute 9 to 12 cc of the ICD volume.

In ICDs, as in other applications where space is a critical designelement, it is desirable to use capacitors with the greatest possiblecapacitance per unit volume. By electrolytically etching aluminum foils,an enlargement of a surface area of the foil will occur withoutenlargement of the overall capacitor. As a result of this enlargement ofthe surface area, electrolytic capacitors can obtain a given capacitywith a smaller volume than an electrolytic capacitor which utilizes afoil with an unetched surface. Likewise, etched-foil capacitors of agiven volume can obtain a higher capacitance compared to unetched-foilcapacitors.

Etching the foil increases the surface area of the foil by roughening anotherwise flat surface. A metal foil can be etched according to anymethod that increases the surface area, such as electrochemical etching,roughening the foil surface mechanically and chemical etching.Electrochemical etching increases the surface area of the foil byelectrochemically removing portions of the foil to create etch tunnels.Electrochemical etching is done according to any known etch process,such as the ones discussed in U.S. Pat. No. 4,474,657 to Arora; U.S.Pat. No. 4,518,471 to Arora; U.S. Pat. No. 4,525,249 to Arora and U.S.Pat. No. 5,715,133 to Harrington et al., each of which is incorporatedherein by reference in its entirety.

In a conventional electrolytic etching process, surface area of the foilis increased by removing portions of the aluminum foil to create etchtunnels. The foil used for such etching is typically an etchablealuminum strip of high cubicity. The etch initiation and hence the gainor capacitance of the foil is the result of several variables, such asfoil cubicity, thermal oxide on the foil, and the electrochemicalreaction.

As tunnel density (i.e., the number of tunnels per square centimeter) isincreased, a corresponding enlargement of the overall surface area willoccur. Larger surface area results in higher overall capacitance.

Creating a pattern on an aluminum foil surface has been previouslydemonstrated as a means to successfully increase surface area. Forexample, U.S. Pat. No. 7,150,767 to Schneider, et al., which isincorporated herein by reference in its entirety, discloses an etchingprocess which applies a holographic image to a photoresist coated on afoil to create a pattern of photoresist on the foil prior to etching.The photoresist pattern on the foil surface allows for positionalcontrol of tunnel initiation. Alternatively, U.S. Pat. No. 6,224,738 toSudduth, et al. and U.S. Pat. No. 6,736,956 to Hemphill et al., whichare incorporated herein by reference in their entirety, disclose etchingprocesses which utilize masking to control tunnel initiation.

By controlling the position of tunnel initiation, foils are etched moreuniformly and have optimum tunnel distributions. The difficulty arises,however, in attempting to control the pattern on a 0.1 to 5 micron (μm)scale. Typical etching processes used to initiate tunnel formation canundercut a patterned resist. Tunneling can begin in directions otherthan orthogonal to the foil surface. This can cause release of thephotoresist, increase the brittleness of the foil, and/or reduce theoptimization of the morphology of the foil surface. There is a needtherefore for a process of foil etching for use in electrolyticcapacitors which allows for improved control of tunnel initiation on amicron or sub-micron level.

SUMMARY OF THE DISCLOSURE

Disclosed herein are methods to improve upon prior etching techniques byoptimizing the morphology and distribution of tunnels in a metal foilsurface. It is important to control the initiation of the tunnels forthe reasons stated above. Tunnel initiation can be controlled throughthe use of a hydrate layer on a metal substrate surface.

Also disclosed herein is a method for patterning a metal substrate, themethod including the steps of: applying a photo-sensitive layer on asurface of a metal substrate; forming a patterned surface on thephoto-sensitive layer; removing at least a portion of the patternedsurface of the photo-sensitive layer to create a patterned resist and aplurality of unmasked areas on the metal substrate; forming a hydratelayer in at least one of the unmasked areas; removing the patternedresist to expose at least one portion of the surface of the metalsubstrate; and etching at least one exposed portion of the surface ofthe metal substrate to initiate tunnels in the metal substrate, therebyforming a patterned metal substrate, wherein the hydrate layer acts as amask during the etching.

Further disclosed herein is a method of manufacturing a patternedaluminum foil for use in an electrolytic capacitor, the methodcomprising the steps of applying a photo-sensitive layer on a surface ofan aluminum foil; forming a patterned surface on the photo-sensitivelayer; plasma etching the patterned surface of the photo-sensitive layerto create a patterned resist and unmasked areas on the aluminum foil;forming a hydrate layer mesh in the unmasked areas; removing thepatterned resist to expose the surface of the aluminum foil; andelectrochemically etching the exposed surface of the aluminum foil toinitiate tunneling of the aluminum foil to form the patterned aluminumfoil surface, wherein the hydrate layer mesh acts as a mask during theelectrochemical etching.

Additionally disclosed herein is a method for patterning a metalsubstrate, the method comprising the steps of: applying aphoto-sensitive layer on a surface of a metal substrate; forming apatterned surface formed of an array of microstructures on the surfaceof the photo-sensitive layer; removing at least a portion of thepatterned surface of the photo-sensitive layer to create (i) a patternedresist having a microstructure array configuration corresponding to anddimensionally proportional to the patterned surface and (ii) at leastone unmasked portion of the surface of the metal substrate; forming ahydrate layer mesh in the at least one unmasked portion of the surfaceof the metal substrate; removing the patterned resist to expose at leastone portion of the surface of the metal substrate; and etching the atleast one exposed portion of the surface of the metal substrate toinitiate tunnels in the metal substrate, thereby forming a patternedmetal substrate, wherein the hydrate layer acts as a mask during theetching.

Also disclosed herein are the patterned metal substrates resulting fromthe methods disclosed herein. For example, a patterned metal substratecan include a metal substrate having a first surface and a secondsurface; a hydrate layer incorporated into the first surface; and aplurality of tunnels extending from the first surface. In someembodiments, the hydrate layer can have a thickness in a range from 0.1to 1 micron. In some embodiments, a hydrate layer can also beincorporated into the second surface of the metal substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated herein and form part of thespecification. Together with the detailed description, the drawingsfurther serve to explain the principles of and to enable a personskilled in the relevant art(s) to make and use the devices presentedherein.

FIG. 1 is a flowchart of illustrating a process for patterning the foilsurface according to an exemplary embodiment.

FIG. 2A is an enlarged, cross-sectional view of an exemplary surfaceafter a step of applying a photo-sensitive layer to a metal foilsubstrate.

FIG. 26 is an enlarged, cross-sectional view of an exemplary surfaceafter a step of applying a mold to emboss the photo-sensitive layer.

FIG. 2C is an enlarged, cross-sectional view of an exemplary surfaceafter a step of curing the photo-sensitive layer and removing the mold.

FIG. 2D is an enlarged, cross-sectional view of an exemplary surfaceafter a step of etching the photo-sensitive layer to form a patternedresist.

FIG. 2E is an enlarged, cross-sectional view of an exemplary surfaceafter a step of forming a hydrate layer on the surface of the metal foilsubstrate.

FIG. 2F is an enlarged, cross-sectional view of an exemplary surfaceafter a step of removing the patterned resist.

FIG. 2G is an enlarged, cross-sectional view of an exemplary surfaceafter a step of etching the metal foil substrate.

FIG. 3 is an enlarged, cross-sectional view of an exemplary surfaceshown in FIG. 2C.

FIG. 4 is a plane view of an exemplary hydrate layer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described with reference to theaccompanying drawings, in which like reference numerals refer to similarelements. While specific configurations and arrangements are discussed,it should be understood that this is done for illustrative purposesonly. A person skilled in the pertinent art will recognize that otherconfigurations and arrangements can be used without departing from thespirit and scope of the present invention. It will be apparent to aperson skilled in the pertinent art that this invention can also beemployed in a variety of other applications.

A series of surface treatments can be performed on a metal foil surfaceto control tunnel initiation on a micron or sub-micron level, forexample on a 100 nm to 5 μm scale, to create, for example, anodes for anelectrolytic capacitor. As tunnel density (i.e., the number of tunnelsper square centimeter) is increased, a corresponding enlargement of theoverall surface area will occur. Larger surface area results in higheroverall capacitance. Thus, controlling tunnel initiation on a micron orsub-micron level to create a periodic array of etch tunnels willincrease the surface area of the foil and raise capacitance by 2.5 to 3times. The foil can be used as anodes, which in turn are used inassembling a capacitor. The increased surface area for the anodesequates to a reduction in overall capacitor volume of about 50-66%,which in terms of current ICD volume is a 9 to 12 cc reduction. Oralternatively, fewer anodes are needed to deliver the same energyoutput, thereby reducing production costs of the capacitor.

Forming a hydration layer can be one of the surface treatments. Thehydration layer can act as a mask while etching the surface of the metalfoil, and thereby can enable control of the tunnel initiation on amicron or sub-micron level, for example on a on a 100 nm to 5 μm scale.In some embodiments, the presence of the hydration layer minimizes, orcompletely eliminates, undercutting during etching. For example theetching process does not weaken the interface between the hydrationlayer and the foil surface. This minimizes the likelihood that etchtunnels will begin in directions other than orthogonal to the foilsurface. As a result, the tunnels can be initiated closer together,thereby increasing the tunnel density without fear that adjacent tunnelswill converge.

As depicted in FIG. 1, a series of steps can be performed to impart apattern on a metal substrate. FIG. 1 is a high level flowchart of anexemplary method for preparing and etching a surface of a metalsubstrate. FIGS. 2A-2G depict an exemplary cross-sectional view of themetal substrate after each step. Though the surface modificationdescribed below is explained regarding a single surface, one of ordinaryskill will appreciate that the process can be repeated, simultaneouslyor sequentially, on an opposing side of the metal substrate to maximizesurface area on both sides of the substrate.

The method can include a step 110 of applying a photo-sensitive layer toa surface of a metal substrate to form a multilayer composite. Asdepicted in FIG. 2A, the multilayer composite can include a metalsubstrate 200 having a first surface 201 and a photo-sensitive layer 202applied to first surface 201 of metal substrate 200. In someembodiments, metal substrate 200 can be in the form of a foil withthickness in the range of 85 to 125 microns, preferably 100 to 115microns. In some embodiments, the etched foil resulting from the processcan be used as an electrode in a capacitor, such as an anode or acathode. As such, in some embodiments, metal substrate 200 can be amaterial typically used as an anode or cathode. Anode and cathode foilscan be made of metal (for example, aluminum), metal alloy, or a metalcomposite material. Aluminum foils are well known in the art and arecommercially available. In some embodiments, the foil is aluminum withpurity of at least about 99.96%, preferably about 99.98-99.99%, aluminumand at least about 80% cubic texture. Alternative materials to aluminuminclude, but are not limited to, zinc, zirconium, tantalum, magnesium,niobium, and alloys of any or all of these metals.

In some embodiments, photo-sensitive layer 202 can be anultraviolet-curable (UV-curable) photopolymer that undergoes UVpolymerization. In some embodiments, a UV curable photopolymer can bespread on first surface 201 of metal substrate 200 to have a thicknessof about 0.1 to 50 μm. Upon drying, the photopolymer can be exposed toUV light to cure it.

The method of forming a patterned surface on a metal substrate caninclude a step 120 of forming a patterned surface 203 on photo-sensitivelayer 202. As shown in FIG. 2B, a mold 204 can be pressed or embossedonto photo-sensitive layer 202 that has been applied to surface 201 ofmetal substrate 200. Mold 204 can have a surface pattern, for example, anano-patterned array of spaced-apart microstructures. After curing thephoto-sensitive layer 202, mold 204 is released from the surface ofphoto-sensitive layer 202 to leave patterned surface 203 onphoto-sensitive layer 202 having an array of microstructurescomplementary in shape to those of mold 204 as shown in FIG. 2C.

Prior to forming patterned surface 203 on photo-sensitive layer 202,mold 204 is prepared. In some embodiments, a patterned sapphiresubstrate serves as a master that can be replicated into mold 204. Thesapphire master has a patterned array of microstructures, e.g.hemispherical bumps on the surface of the substrate. In one embodiment,the microstructures can be in the range of from 0.1 to 5.0 microns indiameter. A releasing agent can be deposited onto the master substrate.This can be accomplished, for example, via silanization with afluorinated molecule as is known in the art. A molding material can thenbe deposited on the substrate to form a complementary nano-pattern onthe surface of a molding material. In some embodiments, the moldingmaterial can be a silicon-based organic polymer, such aspolydimethylsiloxane (PDMS). Upon curing, this molding material can formmold 204.

The microstructures of the sapphire master and/or mold 204 are notlimited to hemispherical bumps. The microstructures can be of anyappropriate shape and configuration, including micro-convexities ormicro-concavities. In some embodiments, the shape of the microstructurescan include, but is not limited to rectangular, hexagonal, conical,frusto-conical, hemispherical, and cylindrical. The microstructures canbe arranged in any appropriate configuration, e.g. aligned or staggeredin columns and rows. Once the mold is imparted with an appropriatesurface morphology, it can be used in conjunction with the methoddescribed herein.

In some embodiments, patterned surface 203 is complementary in shape tothe surface pattern of mold 204. Thus in some embodiments, mold 204imparts patterned surface 203 with a nano-patterned array ofspaced-apart microstructures. In some embodiments, the shape of themicrostructures on patterned surface 203 can include, but is not limitedto rectangular, hexagonal, conical, frusto-conical, hemispherical, andcylindrical. In some embodiments, the microstructures of patternedsurface 203 can have an array microstructures wherein themicrostructures have a width of in the range of 0.1 to 5 μm and/or aperiod (i.e., the center to center distance between adjacentmicrostructures) of about 1 to 3.0 μm. In an example embodiment, themicrostructures have a width of about 1 μm and/or a period of about 1.5μm.

Returning to the method of patterning metal substrate 200, FIG. 3represents a magnified view of FIG. 2C. As shown in FIG. 3, patternedsurface 203 on photo-sensitive layer 202 includes a lower portion with aheight h and an upper portion with a height H. In some embodiments,height h is the minimum height of photo-sensitive layer 202, i.e. heighth can correspond to indented areas of photo-sensitive layer 202 betweenmicrostructures of patterned surface 203. Height H can correspond to themaximum height of surface 203 of photo-sensitive layer 202 minus theminimum height h. In other words, height H can represent the height ofthe microstructures. In some embodiments, the total height H+h ofphoto-sensitive layer 202 can be from about 10 μm to about 50 μm. Insome embodiments, height H is greater than height h.

Next, the method can further include a step 130 of etching patternedsurface 203 to form a patterned resist 205. Patterned surface 203 can beetched uniformly to remove a portion of photo-sensitive layer 202.Etching can include, but is not limited to, plasma etching, for examplewith oxygen plasma. In some embodiments, etching step 130 can be used toreduce photo-sensitive layer 202 down to a patterned resist 205 as shownin FIG. 2D. In some embodiments, photo-sensitive layer 202 can bereduced by a thickness corresponding to height h. This can create apatterned resist in the form of an array of sub-micron bumps or othermicrostructures with first surface 201 of metal substrate 200 exposedaround them. Thus, patterned resist 205 masks a portion of surface 201of metal substrate 200, but leaves areas 207 interspersed throughoutpatterned resist 205 where surface 201 of metal substrate 200 isexposed. In some embodiments, when photo-sensitive layer 202 is reducedby height h, exposed areas 207 correspond to areas between themicrostructures of photo-sensitive layer 202. Thus, in some embodimentspatterned resist 205 can resemble patterned surface 203 in thatpatterned resist 205 has a same array configuration of microstructuresand/or is dimensionally proportional to patterned surface 203.

The method can include a step 140 of forming a hydrate layer 206 inexposed areas 207 of resist 205. As shown, for example, in FIG. 2E,hydrate layer 206 can be formed only in the unmasked areas of metalsubstrate 200 (i.e., exposed areas 207). In some embodiments, hydratelayer 206 can be created by dipping surface 201 of metal substrate 200in a bath of water. Water reacts with the exposed areas of metalsubstrate 200 to form hydrate layer 206 above and below surface 201 ofmetal substrate 200. Hydrate layer 206 thus forms a mask above and belowsurface 201 of metal substrate 200 thereby forming a layer of protectionthat extends below surface 201 of metal substrate 200. In someembodiments, the thickness of hydrate layer 206 is about 0.1 to 1microns.

The following is an exemplary process for creating a hydrate layer whenan aluminum foil is used as metal substrate 200. The aluminum foil canbe dipped in 95° C. deionized (DI) water for 4 to 12 min, preferably 8min, to form aluminum hydroxide above and below the surface in the areasnot protected by patterned resist 205. In some embodiments, thetemperature of the water is in a range from 70 to 100 degrees Celsius.The length of time to form the hydrate layer 206 of a desired thicknessis determined in part by the temperature as the higher the temperature,the shorter the length of time to form a hydrate layer, as would beappreciated by one of ordinary skill in the art.

In some embodiments, the hydration process does not remove patternedresist 205 and only creates hydrate layer 206 in exposed areas 207 inpatterned resist 205. The presence of patterned resist 205 preventsformation of hydrate layer 206 in areas covered by patterned resist 205resulting in hydrate layer 206 forming a mesh or lattice with ahoneycomb configuration in exposed areas 207.

The method can include a step 150 of removing patterned resist 205 toexpose at least one portion of surface 201 of metal substrate 200.Removing patterned resist 205 can include, but is not limited to, usinga non-chemically aggressive agent. Chemically removing the patternedresist 204 leaves behind the hydrate layer mask 206. For example, wherean aluminum foil is elected, a non-chemically aggressive solution toaluminum, such as ethyl lactate, can be used to remove a polymer resist205. The ethyl lactate solution does not remove the hydrate layer, onlythe polymer resist. Thus, as shown in FIG. 2F, the metal substratesurface 201 is masked by the hydrate layer 206, yet exposed in areapreviously covered by patterned resist 205. As depicted in FIG. 4, insome embodiments, hydrate layer 206 can form a mesh or lattice with ahoneycomb configuration around areas of exposed metal substrate surface201. The areas of exposed metal substrate surface 201 can have a widthof about 0.1 to 5 μm and/or a period (i.e., the center to centerdistance between adjacent exposed areas) of about 1 to 3.0 μm. In anexample embodiment, areas of exposed metal substrate surface 201 canhave a width of 1.0 μm and/or a period of about 1.5 μm

The method can include a step 160 of etching metal substrate surface 201to initiate tunnels. In some embodiments, the etching can include, butis not limited to electrochemical etching. Thus, in some embodiments,after hydrate layer 206 is created and patterned resist 204 is removed,metal substrate 200 can be placed in an etching solution known by thoseskilled in the art to electrochemically create tunnels 208 only in theareas of unmasked metal substrate surface 201. As shown in FIG. 2G, apattern of tunnels 208 can be created having substantiallyuniformly-distributed tunnels in metal substrate 200 to maximize asurface area of metal substrate 200. The process outlined in FIG. 1provides a higher etch tunnel density and higher capacitance as a resultof using hydrate layer 206 as a mask when performing etching step 160.Thus, in some embodiments, the methods disclosed herein result in apatterned metal substrate 200, as shown for example in FIG. 2G, having afirst surface 201 and an opposing second surface, a hydrate layer 206incorporated into first surface 201, and a plurality of tunnels 208formed in and extending from first surface 201. In some embodiments, theprocess steps disclosed herein are also formed on a second surface ofmetal substrate 200 such that the second surface also has a hydratelayer incorporated into it.

As discussed above, if a polymer mask is used when etching surface 201,etching step 160 can undercut a polymer mask, which leads to tunnelswhich are not orthogonal to surface 201. As a result, when using apolymer mask the tunnel initiation spots need to be spaced further apartto prevent adjacent tunnels from converging. This does not allow formaximizing the surface area of metal substrate 200. By contrast, thepresence of hydrate layer 206 can minimize, or completely prevent,undercutting of the interface of hydrate layer 206 and metal surface201. Further, the direction of tunnel initiation is controlled at metalsubstrate surface 201 because a portion of hydrate layer 206 has formedbelow metal substrate surface 201. Thus, tunnels initiate orthogonal tometal substrate surface 201 and the portion of hydrate layer 206 belowsurface 201 can prevent the tunnels from angling outward.

In some embodiments, metal substrate 200 is etched according to anymethod that increases the surface area, for example electrochemicaletching, as is known to one skilled in the relevant art. Electrochemicaletching can increase the surface area of metal substrate 200 byelectrochemically removing portions of metal substrate 200 to createetch tunnels.

In some embodiments, surface 201 can be etched in an aqueous halidebased etch solution, preferably a hydrochloric acid or sodium chloridesolution, according to a conventional etch process; for example, U.S.Pat. No. 5,715,133 to Harrington et al, describes a suitable method ofetching foil and is incorporated herein by reference in its entirety.The etch solution preferably consists of about 1.5% by weight sodiumchloride, about 3.0% by weight sodium perchlorate, about 3% by weightglycerin, 0.5% by weight sodium sulfate, and deionized water. The etchsolution preferably is heated to a temperature in the range of about 60°C. to about 95° C., preferably 35° C. The foil is etched at a DC currentdensity of about 0.01 A/cm² to about 0.50 A/cm², preferably about 0.15A/cm². A charge of about 15 coulombs/cm² to 100 coulombs/cm², preferablyabout 30 coulombs/cm², is passed through the foil during the etchingprocess, which requires an etch time in the range of about 2 minutes toabout 12 minutes.

After etching, the foil can be further processed in a combination ofoptional steps such as widening, forming and finishing steps. Forexample, after etching, the foil can be removed from the etch solutionand rinsed in deionized water. The tunnels formed during the initialetch are then widened, or enlarged, in a secondary etch solution,typically an aqueous based nitrate solution, preferably between about 1%to about 20% aluminum nitrate, more preferably between about 10% toabout 14% aluminum nitrate, with less than about 1% free nitric acid.The etch tunnels are widened to an appropriate diameter by methods knownto those in the art, such as that disclosed in U.S. Pat. No. 4,518,471to Arora and U.S. Pat. No. 4,525,249 to Arora, both of which areincorporated herein by reference in their entirety.

After the etch tunnels have been widened, the foil is again rinsed withdeionized water and dried. Finally, a barrier oxide layer can be formedonto one or both surfaces of the metal foil by placing the foil into anelectrolyte bath and applying a positive voltage to the metal foil and anegative voltage to the electrolyte. Preferably, the dielectric oxidelayer is grown anodically in an aqueous formation electrolyte, with aminimum voltage that is 10% greater than the intended working voltage ofthe finished capacitor. The barrier oxide layer provides a highresistance to current passing between the electrolyte and the metalfoils in the finished capacitor, also referred to as the leakagecurrent. High leakage current can result in the poor performance andreliability of an electrolytic capacitor. In particular, high leakagecurrent results in greater amount of charge leaking out of the capacitoronce it has been charged.

The formation process can include applying a voltage to the foil throughan electrolyte such as boric acid and water or other solutions familiarto those skilled in the art, resulting in the formation of an oxide onthe surface of the anode foil. In some embodiments, the electrolyte forformation is a 100-1000 μS/cm, preferably 500 μS/cm, citric acidconcentration. In the case of an aluminum anode foil, the formationprocess results in the formation of aluminum oxide (Al₂O) on the surfaceof the anode foil. The thickness of the oxide deposited or “formed” onthe anode foil is proportional to the applied voltage, roughly 10 to 15Angstroms per applied volt.

The etched and formed anode foils can be cut and used to assemble acapacitor. It is understood to one of ordinary skill in the art thatadditional steps, or an alternative combination of steps, can be used inprocessing an etched anode foil. Additional steps include, but are notlimited to rinsing, drying, hydrating, capacitance testing, annealing,and capacitor assembling. These steps can be carried out using anymethod known to one skilled in the art.

In some embodiments, an etched anode foil can be produced by the methodsdescribed above. Anode foils are punched from etched anode sheets. Foilsheets etched in accordance with the methods described above and theresulting anode foils exhibit high capacitance while maintaining highstrength.

In some embodiments, an electrolytic capacitor can be formed comprisingetched aluminum anode foils, which have been prepared using the methodsdescribed above. Such capacitors can be made using any suitable methodsknown in the art. Non-limiting examples of such methods are disclosed,e.g. in the following references: U.S. Pat. No. 4,696,082 to Fonfria atal., U.S. Pat. No. 4,663,892 to Kenmochi, U.S. Pat. No. 3,872,579 toPapadopoulos, U.S. Pat. No. 4,541,037 to Ross at al., U.S. Pat. No.4,266,332 to Markarian et al., U.S. Pat. No. 3,622,843 to Vermiyea etal., and U.S. Pat. No. 4,593,343 to Ross, each of which is incorporatedherein by reference in its entirety.

Electrolytic capacitors manufactured with anode foils etched accordingto the methods described above can be utilized in ICUs, such as thosedescribed in U.S. Pat. No. 5,522,851 to Fayram, incorporated herein byreference in its entirety. The increased capacitance per unit volume ofthe electrolytic capacitor will allow for a reduction in the size of theICD.

Example

A UV curable photopolymer was spread on the surface of an aluminum foilsheet to a thickness from about 10 to 50 μm. The photopolymer was driedon the surface of the aluminum foil at 50 CC until completely dry (˜10minutes). A 1/16^(th) inch (1.5875 mm) thick template with through-holeswith 1 mm diameters and 2.5 mm center-to-center spacings was placedagainst the foil exposing only the openings to UV light. The exposedareas were cured with UV light.

After the UV light cured the photopolymer in the exposed areas, alcoholwas used to remove the uncured photopolymer. A photopolymer patternmimicking the template was now on the surface of the aluminum foil. Thefoil was placed in 95° C. de-ionized water for 20 minutes to create thehydrate layer in the areas without the photopolymer coverage. The foilwas then placed in a 25° C. ethyl lactate solution for 2 minutes toremove the photopolymer while leaving the hydrate layer intact. The foilwas etched electrochemically to form a etch pattern of tunnels in thealuminum foil. The resulting etch pattern mimicked the original templatepattern.

By controlling the position of tunnel initiation, foils can be etchedmore uniformly and have optimum tunnel distributions. The processesdescribed herein to initiate tunnel formation obviates undercutting ofthe patterned resist and allows for controlling tunnel initiation on amicron or sub-micron scale, for example on a 100 nm to 5 μm scale. Thehydrate layer is incorporated into the metal surface by the hydrationprocess and reduces the possibility of unintentional release. Also,tunneling does not begin in directions other than orthogonal to the foilsurface because the hydrate layer penetrates into and extends below themetal substrate surface. Accordingly, adjacent tunnels do not merge justbelow the foil surface, and therefore avoids increasing the brittlenessof the foil or reducing the optimization of the morphology of the foilsurface associated with adjacent tunnels merging. Controlling tunnelinitiation on a micron or sub-micron scale with the use of theprotective hydration layer mask can increase the surface area of thefoil and raise capacitance by 2.5 to 3 times. The foil can be used asanodes, which in turn are used in assembling a capacitor. The increasedsurface area for the anodes equates to a reduction in overall capacitorvolume of about 50-66% which in terms of current ICD volume is a 9 to 12cc reduction. Or alternatively, fewer anodes are needed to deliver thesame energy output, thereby reducing production costs of the capacitor.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.While the invention has been particularly shown and described withreference to example embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details can be madetherein without departing from the spirit and the scope of theinvention.

What is claimed is:
 1. A method for patterning a metal substrate, themethod comprising the steps of: applying a photo-sensitive layer on asurface of a metal substrate; forming a patterned surface on thephoto-sensitive layer; removing at least a portion of the patternedsurface of the photo-sensitive layer to create a patterned resist and aplurality of unmasked areas on the metal substrate; forming a hydratelayer in at least one of the unmasked areas; removing the patternedresist to expose at least one portion of the surface of the metalsubstrate; and etching at least one exposed portion of the surface ofthe metal substrate to initiate tunnels in the metal substrate, therebyforming a patterned metal substrate, wherein the hydrate layer acts as amask during the etching.
 2. The method of claim 1, wherein the metalsubstrate is an aluminum foil.
 3. The method of claim 1, wherein thephoto-sensitive layer is an ultraviolet-curable photopolymer.
 4. Themethod of claim 1, wherein the patterned surface comprises an arraymicrostructures and the microstructures have a width of in a range from0.1 to 5 μm and a period in a range from 1 to 3 μm.
 5. The method ofclaim 1, wherein the step of removing at least a portion of thepatterned surface comprises etching the patterned surface with areactive plasma.
 6. The method of claim 1, wherein the metal substratehas a first surface and a second surface and the method of claim 1 isperformed on both the first surface and the second surface.
 7. Themethod of claim 1, wherein the patterned resist has the same arrayconfiguration and is dimensionally proportional to the patternedsurface.
 8. The method of claim 1, wherein the step of forming a hydratelayer comprises reacting deionized water with the substrate surface toform a metal hydrate.
 9. The method of claim 1, wherein the hydratelayer forms a mesh on the surface of the metal substrate leaving exposedmetal substrate areas that have a width of in a range from 0.1 to 5 μmand a period in a range from 1 to 3 μm.
 10. A method of manufacturing apatterned aluminum foil for use in an electrolytic capacitor, the methodcomprising the steps of: applying a photo-sensitive layer on a surfaceof an aluminum foil; forming a patterned surface on the photo-sensitivelayer; plasma etching the patterned surface of the photo-sensitive layerto create a patterned resist and unmasked areas on the aluminum foil;forming a hydrate layer mesh in the unmasked areas; removing thepatterned resist to expose the surface of the aluminum foil; andelectrochemically etching the exposed surface of the aluminum foil toinitiate tunneling of the aluminum foil to form the patterned aluminumfoil surface, wherein the hydrate layer mesh acts as a mask during theelectrochemical etching.
 11. The method of claim 10, wherein thephoto-sensitive layer is an ultraviolet-curable photopolymer.
 12. Themethod of claim 10, wherein the patterned surface comprises an array ofmicrostructures and the microstructures have a width of in a range from0.1 to 5 μm and a period in a range from 1 to 3 μm.
 13. The method ofclaim 10, wherein the aluminum foil has a first surface and a secondsurface and the method of claim 11 is performed on both the firstsurface and the second surface.
 14. The method of claim 10, wherein thepatterned resist has the same array configuration and is dimensionallyproportional to the patterned surface.
 15. The method of claim 10,wherein the step of forming a hydrate layer mesh comprises reactingdeionized water with the substrate surface to form metal hydrate. 16.The method of claim 10, wherein the hydrate layer mesh forms a honeycombconfiguration on the surface of the aluminum foil leaving exposedaluminum foil areas that have a width of in a range from 0.1 to 5 μm anda period in a range from 1 to 3 μm.
 17. The method of claim 10, whereinthe aluminum foil is used as an anode or cathode in the electrolyticcapacitor.
 18. A method for patterning a metal substrate, the methodcomprising the steps of: applying a photo-sensitive layer on a surfaceof a metal substrate; forming a patterned surface formed of an array ofmicrostructures on the surface of the photo-sensitive layer; removing atleast a portion of the patterned surface of the photo-sensitive layer tocreate (i) a patterned resist having a microstructure arrayconfiguration corresponding to and dimensionally proportional to thepatterned surface and (ii) at least one unmasked portion of the surfaceof the metal substrate; forming a hydrate layer mesh in the at least oneunmasked portion of the surface of the metal substrate; removing thepatterned resist to expose at least one portion of the surface of themetal substrate; and etching the at least one exposed portion of thesurface of the metal substrate to initiate tunnels in the metalsubstrate, thereby forming a patterned metal substrate, wherein thehydrate layer acts as a mask during the etching.
 19. The method of claim18, wherein the step of removing the patterned resist comprises placingthe substrate in an ethyl lactate solution to remove the patternedresist while leaving the hydrate layer mesh intact.
 20. The method ofclaim 18, wherein the aluminum foil has a first surface and a secondsurface and the method of claim 18 is performed on both the firstsurface and the second surface.
 21. A patterned metal substratecomprising: a metal substrate having a first surface and a secondsurface; a hydrate layer incorporated into the first surface; and aplurality of tunnels extending from the first surface.
 22. The patternedmetal substrate of claim 21, further comprising a hydrate layerincorporated into the second surface.
 23. The patterned metal surface ofclaim 21, wherein the hydrate layer has a thickness in a range from 0.1to 1 micron.
 24. The patterned metal surface of claim 21, wherein themetal substrate is an aluminum foil.